Work one growth and pattern formation published in Science

Regulation of growth and pattern formation

How the development of a complete organism from initially a few identical cells is regulated is one of the fundamental questions of biology. In a joint project between the groups of Dolf Weijers (Laboratory of Biochemistry) and Christian Fleck (Laboratory of Systems and Synthetic Biology) we investigated the mechanisms underlying the development of the plant embryo. While early development of animals is characterized by cell migration, plant cells are immobile and tightly connected to each other. As a consequence, the plant embryo mainly grows through strictly oriented cell divisions in three dimensions. At the same time these groups of cells need to acquire specific ‘identities’ that will eventually lead to the formation of the whole plant. Up to now, it was completely unclear how these two crucial processes of growth and pattern formation were controlled during tissue formation in a way that the tissue remains stable despite continuous cell divisions. A network switches on a set of genes that cause the production of the plant hormone cytokinin that, in its turn, regulates cell division and the orientation of those divisions. We discovered that pattern formation of the vascular tissues already takes place when the embryo only contains four vascular precursor cells. Theoretical considerations predicted that two of these cells must share a small piece of cell wall. Re-examination of the microscopical images confirmed this prediction even in old recordings from 1995. Interestingly, nobody had ever noticed this, until we pointed out that theoretically this connection must exist.

The four-cell stage is therefore not a simple clump of identical cells. The secret of patterning is thus the combination of a common cell wall connection and a slight difference in concentration of the plant hormone auxin. The genetic circuit that was discovered by these researchers than makes sure the four cells further develop into a complete vascular tissue containing distinct cell types. The circuit consists of two connected incoherent feed forward loops, each of which performs specific tasks. Together they control both growth through oriented cell divisions and pattern formation leading to distinct cell types.